Charge transport, doping and luminescence in solution-processed, phosphorescent, air-stable tellurophene thin films

Recently synthesized small molecule tellurophenes containing ring-appended pinacolboronate (BPin) side groups possess remarkable guest-free air-stable solid-state phosphorescence, structure-based color-tunability and aggregation induced enhanced emission. The charge transport, doping and luminescence behavior of thin transparent films of a tellurophene with BPin groups positioned at the 2,5-positions (B-Te-6-B) was investigated. Film formation played a critical role in determining the hole mobility and the photoluminescence (PL) lifetime. Drop-coated films showed the strongest crystallinity, the highest PL quantum yields and a hole mobility (μ_p) of 1.1 × 10⁻⁴ cm² V⁻¹ s⁻¹, which places tellurophenes in a select group of high mobility phosphorescent emitters. B-Te-6-B was also found to spontaneously form high aspect-ratio microwires upon drop-casting from supersaturated solutions. Oxidative doping in solution by a N(C₆H₄Br)₃[SbCl₆]/LiNTf₂ reagent combination (Tf = SO₂CF₃) increased conductivity by 2-4 orders of magnitude without inducing a color change in the films, while exposure to iodine vapor induced a dramatic change in color together with a 4-6 order of magnitude change in the conductivity. The optical transparency, facile electrical doping and relatively high hole mobilities of B-Te-6-B in solution processed thin films offer promise for the use of tellurophenes as host-free emissive layers and hole transport layers in organic optoelectronic devices.